1. Technical Field
The present disclosure relates to an SR motor control apparatus.
2. Related Art
An SR motor includes a U-phase coil, a V-phase coil, and a W-phase coil. These coils are connected in a star connection. This starconnecting node (the node at which terminals of the three coils are coupled) is a neutral node of the SR motor. An SR motor control apparatus supplies a control current to such an SR motor. This type of the SR motor control apparatus mainly includes a control board and a switching circuit. The control board outputs a control signal for controlling a current supplied from a power source to the SR motor. The switching circuit is connected to the positive electrode and the negative electrode of the power source. Furthermore, the switching circuit is connected to the neutral node of the SR motor, the U-phase coil, the V-phase coil, and the W-phase coil. The switching circuit supplies the control current to the coils of the SR motor in response to control signals from the control board.
The above-described SR motor control apparatus is disclosed in, for example, Japanese Patent Application Laid-Open No. 2007-28866. The switching circuit of the control apparatus disclosed in this document has four half-bridge circuits. Each of these half-bridge circuits has two power MOSFETs connected in series at the connecting node. These half-bridge circuits are arranged in parallel to each other with respect to the power source. In the half-bridge circuit, the source terminal of one FET and the drain terminal of the other FET are connected in series. The drain terminal of the one FET is connected to the positive electrode of the power source. The source terminal of the other FET is connected to the negative electrode of the power source.
The neutral node of the SR motor is connected to the connecting node of one of the four half-bridge circuits, or a first half-bridge circuit. Also, the U-phase coil, the V-phase coil, and the W-phase coil are connected to the connecting nodes of the rest three half-bridge circuits, respectively.
Here, when the SR motor operates, two FETs of the first half-bridge circuit are switched such that either one of them is almost always in a turned-on state. Therefore, a current almost always flows in either one of these FETs, except for a dead time period in the switching operation.
On the other hand, in six FETs of the rest of three half-bridge circuits, the FETs are sequentially switched one by one based on Pulse Width Modulation (PWM) signals. Therefore, the current sequentially and intermittently flows in these half-bridge circuits.
That is, the time period of the turned-on state in the FET of the first half-bridge circuit is longer than that in the FET of the rest of three half-bridge circuits. Thus, the time period when the current flows in the FET of the first half-bridge circuit is relatively long.